Solid oxide fuel cells are one representative of fuel cells. A cell used in these fuel cells generally has a laminated body with a three-layered structure including a thin solid electrolyte layer formed of a sintered body made for example of yttria-stabilized zirconia (YSZ), a fuel electrode stacked on one surface side of the solid electrolyte layer, and an air electrode stacked on an opposite surface side of the solid electrolyte layer. For example, a cermet of Ni and YSZ is used to form the fuel electrode and lanthanum strontium manganite (LSM) is used to form the air electrode. Both of these electrodes are porous sintered bodies.
To run the solid oxide fuel cell, hydrogen-rich reformed gas, obtained by preheating and reforming hydrocarbon-based raw fuel such as natural gas or LPG, is supplied as fuel gas toward the fuel electrode of the cell while preheated air is supplied as oxidized gas toward the air electrode of the cell under a high-temperature condition from 700 to 1000° C. This generates electromotive force between the fuel electrode and the air electrode of the cell. This electromotive force is generated with a low voltage of 1 V or less. Thus, in the case of a cell like a flat plate, multiple cells are stacked in a thickness direction and connected in series to be used as a cell stack.
Regarding a method of the reforming of raw fuel mentioned herein, the following three types are known. The first type is a steam reforming method using an endothermic catalytic reaction to reform hydrocarbon-based raw fuel containing methane (CH4) such as city gas as a principal component to hydrogen-rich reducing gas with steam. The second type is a partial oxidization reforming method using an exothermic catalytic reaction to also reform hydrocarbon-based raw fuel to hydrogen-rich reducing gas by means of partial oxidation with air. The third type is a method using steam reforming and partial oxidation reforming in combination. This method is to produce thermal independence by combining the former endothermic reaction and the latter exothermic reaction. In terms of the power generation efficiency of a fuel cell system, steam-reformed gas generated by the first method is considered to be preferable.
In evaporators generally used for the generation of steam for steam reforming, water is supplied into a heating chamber heated from the outside with radiant heat from a cell stack or heat of combustion of exhaust gas released from the cell stack (unused gas called off-gas), for example, to evaporate the water. One of these evaporators is described in patent literature 1. The evaporator described in patent literature 1 generates steam by supplying water onto an inclined heating surface in a heating chamber heated from the outside with the heat of combustion of exhaust gas released from a cell stack and evaporating the water.
One of the problems in such an evaporator for fuel cells is reduction in steam generation efficiency due to the Leidenfrost phenomenon that occurs for the following reason. Regardless of whether a heat source is the radiant heat from the cell stack or the energy of the exhaust gas from the cell stack, the temperature of the heat source is as high as several hundred degrees C. This places the heating surface in the heating chamber in a temperature from 200 to 300° C. in many cases, by which the Leidenfrost phenomenon occurs. The following explains a problem relating to reduction in steam generation efficiency due to the Leidenfrost phenomenon.
FIG. 5 is a graph showing the behavior of a water droplet on the heating surface. This graph shows a relationship between the temperature of the heating surface and the lifetime of a water droplet on the heating surface. A region of the temperature of the heating surface up to about 110° C. is a non-boiling region where a droplet on the heating surface wets the heating surface and is evaporated. Thus, the lifetime of the droplet is reduced rapidly with an increase in the temperature of the heating surface. A region of the temperature of the heating surface from 110 to 160° C. is a core boiling region where the droplet on the heating surface spreads largely on the heating surface, boils rapidly, and then disappears. The core boiling region is a temperature region of the highest evaporation efficiency.
By contrast, in a region of the temperature of the heating surface from 160 to 300° C., a water droplet on the heating surface is divided into several pieces and these pieces behave in a manner such as to dance on the heating surface. This is the Leidenfrost phenomenon. At this time, the lifetime of the droplet on the heating surface increases with increase in the temperature of the heating surface. At a maximum, this lifetime increases to a degree substantially the same as its lifetime when the temperature of the heating surface is several tens of degrees C. As the temperature of the heating surface is increased to a temperature higher than 300° C., the droplet comes to rest while maintaining its shape as a rotary ellipsoidal body. This is called a spheroidal state where the lifetime of the droplet is reduced with increase in the temperature of the heating surface.
As understood from the above, the evaporator encounters reduction in steam generation efficiency if the temperature of the heating surface is in a temperature region from 200 to 300° C. where the lifetime of a droplet is extended notably by the Leidenfrost phenomenon. In a higher temperature region, the lifetime of the droplet extended once by the Leidenfrost phenomenon is still influential, and steam generation efficiency continues to be low. Thus, in terms of steam generation efficiency, it is important to make sure the temperature of the heating surface does not reach such temperature regions.
However, in terms of a relationship with the temperature of a heat source, avoiding these temperature regions is difficult in an evaporator for fuel cells. Even if these temperature regions can be avoided, the temperature of the heat source becomes too low. This in turns makes the degradation of start-up characteristics unavoidable at the start of running. Similarly, heat insulation of the heating chamber is effective in terms of reducing the temperature of the heating surface, but this also causes deterioration of start-up characteristics at the start of running as it hinders temperature increase of the heating surface at the start of running. In consideration of start-up characteristics at the start of running, the heating surface in the heating chamber should be heated rapidly while the heat source, specifically, atmosphere outside the heating chamber is placed at a high temperature.
The aforementioned reduction in steam generation efficiency due to the Leidenfrost phenomenon is also considered by the evaporator described in patent literature 1. Supplying water onto the inclined heating surface in the heating chamber is intended to be responsive to this reduction. Specifically, if water is supplied onto the inclined heating surface in the heating chamber, the water flows down along the inclined heating surface to suppress the generation of droplets, thereby suppressing the occurrence of the Leidenfrost phenomenon. This evaporator further takes a measure to enhance wettability of a heating surface in a heating container to water on the heating surface by forming fine recesses and projections on this heating surface.
As described above, however, all of these measures require rapid heating of the heating surface to a high temperature and do not allow reduction in the temperature of the heating surface. Thus, these measures cannot be radical solutions.